remoteproc: introduce rproc_get_by_phandle API

Allow users of remoteproc the ability to get a handle to an rproc by
passing a phandle supplied in the user's device tree node. This is
useful in situations that require manual booting of the rproc.

This patch uses the code removed by commit 40e575b1

 ("remoteproc:
remove the get_by_name/put API") for the ref counting but is modified
to use a simple list and locking mechanism and has rproc_get_by_name
replaced with an rproc_get_by_phandle API.

Signed-off-by: Dave Gerlach <d-gerlach@ti.com>
Signed-off-by: Suman Anna <s-anna@ti.com>
[fix order of Signed-off-by tags]
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>